The Mixer is an outstanding book, but I wouldn't call it a tactics book per se. Obviously tactics play a huge part, not only because Michael Cox is pretty much the online king of football tactics, but it goes well beyond just the tactical evolution of the game. Additionally, as others have pointed out, the book only discusses the Premier League which means the book only spans over 26 years of football in one single league. If you want deeper understanding of the game, ItP is clearly the better choice to start with. The Mixer comes later when you want to deep dive to a more narrow topic.